ktls: Use COUNTER_U64_DEFINE_EARLY

This makes it a bit more straightforward to add new counters when
debugging. No functional change intended.

Reviewed by: jhb
Sponsored by: Ampere Computing
Submitted by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D28498

(cherry picked from commit 1755b2b9891bb1bfa7a58383ef5126821f7e46e3)